1. Field of Invention
The present invention relates to shredders for destroying articles, such as documents, CDs, etc.
2. Description of Related Art
Shredders are well known devices for destroying substrate articles, such as documents, CDs, floppy disks, etc. Typically, users purchase shredders to destroy sensitive articles, such as credit card statements with account information, documents containing company trade secrets, etc.
A common type of shredder has a shredder mechanism contained within a housing that is removably mounted atop a container. The shredder mechanism typically has a series of cutter elements that shred articles fed therein and discharge the shredded articles downwardly into the container. The shredder typically has a stated capacity, such as a number of sheets of paper (typically of 20 lb. weight) that may be shredded at one time; however, the feed throat of a typical shredder can receive more sheets of paper than the stated capacity. A common frustration of users of shredders includes feeding too many papers into the feed throat, only to have the shredder jam after it has started to shred the papers. To free the shredder of the papers, the user typically reverses the direction of rotation of the cutter elements via a switch until the papers become free. Occasionally, the jam may be so severe that reversing may not free the paper entirely, and the paper must be pulled out manually, which may be difficult with the paper bound between blades of the cutter elements. In some cases, when article(s) are inserted into the shredder that are too thick or irreversible, the shredder may be overloaded or overheated, and the motor of the shredder mechanism may stall and thus shut down.
In order to prevent such motor stall, some existing designs use other detection devices to anticipate a motor's current-limit. For example, such designs may include load meters (readings based on motor current), speed-based jam detectors, hall effect sensors (for reading motor speed), or other types of speed sensors (e.g., provided on the cutter shafts). In some existing cases, detection of possible overload or motor stall may be prevented by reversing the motor when the system becomes jammed. U.S. Pat. No. 4,495,456, entitled “Automatic Reversing System for Shredder,” illustrates an example of such a machine.
Some shredders may employ a stall or overload detection circuit which monitors a motor's current draw to determine maximum capabilities of the shredding machine and to determine if/when a motor might stall. In such shredders, the idea is to prevent the motor from going into or remaining in a stall condition which not only draws excessive current, but also heats the motor prematurely. Traditionally, these circuits either have a delay or a limiting device (e.g., a thermistor) or have software to ignore the initial in-rush current drawn by the motor to prevent false positive reactions (for possible stalls or short-circuits).
For example, a first known traditional method for setting the overload detection threshold includes setting a fixed value close to the stall current of a machine at its cold state, and then determining if a motor's current draw is close to the fixed value (i.e., using a comparator) during operation. This first method may be effective on a “cold” motor, i.e., a motor that is not running The overload detection circuitry of this type of shredder will only trigger when the motor is stalled/about to stall (i.e., drawn current is close to the fixed value). However, as a motor heats during use, the amount of current being drawn by the motor tends to decrease, and AC fluctuations may occur. This first method is unable to track any decrease in drawn current as the motor heats or fluctuations. This means that a “hot” (i.e., working or rotating) motor will often stall before or without the overload detection circuitry detecting the event.
A second known method for overload detection is a calibration that is performed at the factory or during manufacture, in which a threshold of a shredder is adjusted to a specific load (e.g., sheet count) on that specific machine. This second method may be effective at preventing a user from operating above the ratings of the machine (before stalling), but it, too, can also not track variances in drawn motor current due to heat and/or AC fluctuations. This, in turn, causes the initial threshold to fluctuate, which can either allow excessive load on the system, or it can prematurely limit the user from operating the machine within its capabilities.
In other designs, assumptions have to be made using software based on time, or an extra thermal device has to be added to the motor to track motor temperature. For example, assumptions of the current thermal condition of the motor (and therefore the maximum load) could be approximated by a software algorithm. Such assumptions generally assume that all of the motors in mass-production have similar thermal characteristics and that the efficiencies of the cutting blocks are similar. However, such assumptions are generally incorrect. Although two motors (of the same model) can have similar measured temperatures, this does not equate to them having the same performance characteristics. Variances in material and assembly can change this relationship, for example. In addition, variations in line voltage and frequency are not generally accounted for. This can significantly impact the performance of the motor and impact the stall current reading relative to a fixed threshold.
As noted above, the inrush current initially drawn by a motor when a shredder mechanism is turned on is ignored in prior designs to prevent false readings of overload. However, as described further herein, this disclosure determines and uses this inrush current to determine parameters related to the motor as well as occurrences at which the motor will stall (e.g., due to a jam in the shredder).